It is known that atmospheric air contains compounds which must be eliminated before the introduction of the air into heat exchangers of the cold box of an air separation unit, particularly the compounds carbon dioxide (CO.sub.2), water vapor (H.sub.2 O) and/or hydrocarbons.
Thus, in the absence of such pretreatment of the air to eliminate these impurities of carbon dioxide and water, there takes place a condensation and a solidification of these impurities during cooling of the air to cryogenic temperatures, from which can result problems of blocking the equipment, particularly the heat exchangers, the distillation columns, etc.
Moreover, it is also customary to eliminate the hydrocarbon impurities likely to be present in air, so as to avoid a high concentration of them in the bottom of the distillation column or columns, thereby to avoid the risk of explosion.
At present, this pretreatment of the air is carried out, as the case may be, by the Temperature Swing Adsorption (TSA) process or by the Pressure Swing Adsorption (PSA) process. By the PSA process is meant PSA processes properly so-called, vacuum swing adsorption (VSA) processes, VPSA processes, and the like.
Conventionally, a TSA process cycle for the purification of air comprises the following steps:
a) purification of the air by adsorption of the impurities at superatmospheric pressure and ambient temperature, PA1 b) depressurization of the adsorber to atmospheric pressure or below atmospheric pressure, PA1 c) regeneration of the adsorbent at atmospheric pressure, particularly with residual gases or waste gases, typically impure nitrogen from an air separation unit and reheated to a temperature conventionally between 100 and 200.degree. C. by means of one or several heat exchanges, PA1 d) cooling the adsorbent to ambient temperature or below, particularly by continuing to introduce into it said residual gas from the air separation unit, but not reheated, PA1 e) repressurization of the adsorber with purified air from for example another adsorber operating in the production phase. PA1 in which at least one bed of ballast comprising at least one ballast material having a density higher than the density of the first adsorbent material, is located downstream of the first adsorbent bed, the at least one ballast bed exerting a mechanical pressure on at least one portion of at least the first adsorbent bed. PA1 the at least one bed of ballast and the at least one first adsorbent bed are separated by a loose grill; PA1 the ballast material is an inert or adsorbent material, preferably the ballast material is an inert alumina; PA1 the density of the ballast material is at least twice that of the density of at least the first adsorbent material, preferably at least three times greater; PA1 at least one second bed of adsorbent comprising at least one second adsorbent material, is arranged upstream and/or downstream of the first adsorbent bed, preferably upstream of the first adsorbent bed; PA1 the volume mass of the second adsorbent material is greater than or equal to that of the first adsorbent material; PA1 the heat capacity of the ballast material is less than or equal to the heat capacity of the adsorbent material, preferably less than about 0.24 Kcal/Kg.degree. C.; PA1 the granulometry of the particles of ballast material is greater than the granulometry of the particles of the first adsorbent material and/or of the second adsorbent material; PA1 the second adsorbent material is an activated alumina and/or the first adsorbent material is zeolite, preferably zeolite X having an Si/Al ratio below 1.15, exchanged or not; PA1 the gas flow to be purified is air; PA1 the elimination of CO.sub.2 and of water vapor is carried out in at least one adsorber and, preferably, in at least two adsorbers operating in an alternate fashion; PA1 the process is either TSA or PSA; PA1 operation is carried out at an adsorption pressure of 10.sup.5 to 10.sup.7 Pa; PA1 the process comprises at least one step of cryogenic separation of at least one portion of the purified air, preferably a step of cryogenic distillation of the purified air; PA1 the purification of the gas, particularly of air, is carried out at a temperature comprised between -40.degree. C. and +80.degree. C.
Usually, a PSA process cycle for the purification of air comprises itself substantially the same steps a), b) and e), but is distinguished from the TSA process by the absence of reheating of the residual gases or gases during the regeneration step (step c)), hence the absence of step d) and, in general, a shorter cycle time than the TSA process.
Generally speaking, the devices for pretreatment of air comprise two adsorbers, functioning in an alternate manner, which is to say that one of the adsorbers is in the production phase while the other adsorber is in the regeneration phase.
Such TSA processes for the purification of air are particularly described in U.S. Pat. No. 3,738,084 and French 7725845.
In general, the elimination of CO.sub.2 and water vapor is carried out in one or several adsorbent beds, preferably several adsorbent beds, namely generally a first adsorbent adapted to separate preferentially water, for example a bed of activated alumina, of silica gel or of zeolites, and a second adsorbent bed to remove preferentially CO.sub.2, for example a zeolite. There could be cited particularly U.S. Pat. Nos. 5,531,808, 5,587,003 and 4,233,038.
To obtain effective elimination of CO.sub.2 and water vapor contained in the air in a same and single adsorbent bed is not an easy thing, because the water has an affinity for the adsorbents substantially greater than that of the CO.sub.2. In other words, the selectivity of the adsorbents is more favorable to water than to CO.sub.2.
Moreover, to be able to regenerate an adsorbent saturated with water, it is usual to bring this adsorbent to a regeneration temperature greater than 100.degree. C.
However, very few adsorbents used at present on an industrial scale in TSA units have a physico-chemical structure suitable to resist for a long time such hydrothermal treatment; the alumina type materials are of this type.
There could be cited U.S. Pat. No. 5,232,474, which discloses the use of activated alumina to dry and decarbonize the air by a PSA process.
Conventionally, this adsorbent bed or these adsorbent beds are inserted in an adsorber or adsorbers, also called "bottles".
There exist at present several different geometries for these adsorbers.
In the conventional adsorbers, the beds of the adsorbent are placed horizontally and are stacked one above the other, which is to say that they are superposed, and are separated or not by a separation region or by an "empty" space or a space free from adsorbent material. This arrangement of superposed beds is simple and trouble-free.
On the other hand, the two ends of these adsorbers, which is to say their bottom and their top, have a diameter limited by the technological limits of metalworking and the size to be transported.
Moreover, this type of arrangement does not permit treating high flow rates of gas, for example flow rates of at least 100,000 Nm.sup.3 per hour, because the speed of the gas becomes too high, which gives rise to large pressure drops and an unacceptable and harmful fluidization of the adsorbent bed or beds.
So as to overcome these problems, new adsorbers have been developed, which are designed to treat high flow rates of gas, namely adsorbers with concentric beds, so-called "layered" adsorbers or so-called "superposed" adsorbers. These types of adsorbers are particularly described in EP-A-O 714 689, U.S. Pat. No. 5,447,558, DE-A-39 19 750, FR-A-2541588 and U.S. Pat. No. 4,627,856.
However, given their more complicated technology, these adsorbers require greater investment, which is not always acceptable from an industrial standpoint.
So as to attempt to decrease the costs of the purification processes, researches have been carried out aiming to improve the properties of the adsorbents used, in particular their adsorption capacity.
In this connection can be cited the documents U.S. Pat. Nos. 3,885,927, 4,493,715, EP-A-O 766 991, U.S. Pat. No. 5,587,003, EP-A-O 733 393, U.S. Pat. No. 5,531,808 and EP-A-O 718 024, which propose different types of adsorbents having improved adsorbing capabilities for CO.sub.2, which permits increasing the specific air flow rate treated for a given quantity of adsorbent and hence to decrease, on the one hand, the quantities of adsorbent used, and, on the other hand, the overall costs of the process.
However, the problem of fluidization of the adsorbent bed or beds contained in an adsorber with superposed beds has only been partially solved until now.
Thus, this problem of fluidization can be decreased by increasing the granulometry of the particles of adsorbent.
However, this solution cannot be considered as completely satisfactory because it is opposed to the present tendency, which is to decrease the purification cycle time so as to limit the quantity of adsorbent to be used.
Moreover, the adsorbents must work more and more dynamically, which is to say in the front zone. But a higher granulometry of the particles of adsorbent militates against this adsorbent dynamic, which is not acceptable.
Moreover, the technique of reverse flow cannot be considered either as satisfactory, to the extent to which oil condensates from the gas and water compression are not removed and hence the adsorbent bed is quickly degraded.